Acclimatisation
By Dr Beth Hall-Thompson
Acclimatisation describes a spectrum of physiological responses to the reduced oxygen available to the blood at altitude. Less oxygen is available to the body the higher you get because there is less atmosphere generating pressure to push it into the bloodstream. At Everest Base Camp height there is approximately half the oxygen available at sea-level; and that reduces to less than a third on the summit.
But obviously some individuals bodys’ are able to adapt to this: Reinhold Messner and Peter Habeler stood on top of Everest first without the aid of oxygen in 1972 (for good measure, Messner then repeated it from the Tibetan side in 1980!)
For us mere mortals – there is a range of altitude at which we start to notice the symptoms of altitude – for some it will start at 2000m, the height of many ski resorts, others will not notice until 3000m, but all will be symptomatic at 3,500m if we have abruptly arrived at that height. However the majority of people can ‘acclimatise’ to higher levels by allowing the body time to adjust to reduced oxygen content.
The body takes time to acclimatise; it first responds by increasing the breathing rate, so taking in more oxygen by repetition, but the heart then has to pump faster to distribute it to the tissues. When breathing faster, the climber blows off more carbon dioxide (an acid waste product) from the lungs and so pushes the system towards alkalinity. This excess alkalinity is compensated for by peeing out bicarbonate (an alkali); this increased urination is a beneficial adaptive mechanism, if rather disturbing to sleep, and frustrating in the cold! The body’s homeostatic mechanisms have to work hard to balance the situation.
This imbalance accounts for one of the scary effects – you wake up and your tent buddy appears to have stopped breathing! Suddenly he gasps, takes a few fast breaths and either relaxes again (used to it); or tells you a tale of how he woke up during a dream in which he had been suffocated by avalanche. This is because when asleep we need a build up of CO2 to provoke a breath, and so we seesaw between the two conditions of over and under breathing; called periodic breathing – this is not altitude sickness.
The body does undergo some longer term changes to adapt to the lower oxygen levels; it produces more red blood cells to pick up the available oxygen in the lungs and carry it to the demanding muscles. Although this starts immediately, the full value is not realised for a few days or until we stop ascending. I would expect each of us at Advanced Base Camp, once acclimatised, to have 30-50% more red blood cells than I do here in Lancaster.
The other beneficial long term adaptation is an increase in the number of capillaries within muscle beds; to try and get the oxygen more directly to the areas needing it.
As climbers and trekkers we have to give the body time to adapt; we should chose to take the ‘train high, sleep low approach’ all the way from Kathmandu! The trick is never to sleep much more than 300m above the previous night; but climbing higher during the day, then coming down to sleep should aid the process of acclimatisation. And where possible once 1000m has been ascended, two nights at the same altitude is advised to allow the body to ‘catch up’. This all works up to about 7000m but then the ‘death zone’ effects start to kick in – i.e. such chronic low oxygen becomes debilitating to the body. There is a reason there are no permanent habitations above 5,300m!
As a doctor in the mountains my other mantra (which I will have to attribute to a friend, Sam Marshall) is ‘first out, last in’. Meaning, take your time trekking upwards; the longer you take the more chance you give your body. Over-exertion & dehydration increase the likelihood of suffering from AMS.
Initially we thought that young men had more propensity to suffer AMS as they continue to fall ill in higher proportions; there is now a large body of evidence that suggests it is more likely to be because they tend to push themselves physically.
In fact the good news is that altitude is the great leveller: it is primarily related to genetics and not physical fitness, age or gender; and so by ‘obeying’ the above rules we can all maximise our potential to climb higher comfortably. The bad news is that if you have suffered once, you are likely to suffer again, and at a similar altitude – you have reached your genetic ceiling. As yet we have no evidence based way of predicting who and when someone will suffer.